NEW YORK/WASHINGTON, Jan 2 (Reuters) - Constellation Energy has been awarded a record $1 billion in contracts to supply nuclear power to the U.S. government over the next decade, the company said on Thursday. Constellation, the country's largest operator of nuclear power plants, will deliver electricity to more than 13 federal agencies as part of the agreements with the U.S. General Services Administration. The deal is the biggest energy purchase in the history of the GSA....